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Abstract:

An optical DQPSK modulator comprises a pair of phase modulators. Each of
the pair of the phase modulators is provided with first and second
driving signals. The first and second driving signals are amplified by
first and second amplifiers, respectively. An RZ intensity modulator
generates an optical RZ-DQPSK signal from an optical DQPSK signal output
from the optical DQPSK modulator. A photodetector generates a monitor
signal from the optical RZ-DQPSK signal. A gain adjuster unit adjusts the
gains of the first and second amplifiers so as to minimize the power of
the monitor signal.

Claims:

1. An optical transmitter apparatus, comprising: a phase modulator,
having a first phase modulation element configured to generate a first
modulation signal on a first arm in accordance with a first driving
signal, a second phase modulation element for generating a second
modulation signal on a second arm in accordance with a second driving
signal, and a phase shifter configured to provide a phase difference
between the first arm and the second arm, which generates a
phase-modulated signal by combining the first and second modulation
signals; an intensity modulator configured to perform intensity
modulation on the phase-modulated signal output from the phase modulator;
a photodetector configured to generate a power signal representing a
power of an AC component of an optical signal output from the intensity
modulator; first and second amplifiers configured to amplify the first
and second drive signals; a gain adjuster configured to generate a gain
adjusting signal adjusting again of the first and second amplifiers; a
first superimposer configured to superimpose a first low-frequency signal
on the gain adjusting signal; a first detector configured to perform
synchronous detection on the power signal by using the first
low-frequency signal; a phase adjuster configured to generate a phase
adjusting signal to adjust the amount of phase sift of the phase shifter;
a second superimposer configured to superimpose a second low-frequency
signal on the phase adjusting signal; and a second detector configured to
perform synchronous detection on the power signal by using the second
low-frequency signal, wherein the gain adjuster adjusts a gain of at
least one of the first amplifier and the second amplifier based on a
result of the synchronous detection by the first detector, and the phase
shift adjuster adjusts the amount of phase shift of the phase shifter
based on a result of the synchronous detection by the second detection
unit.

2. An optical transmitter apparatus, comprising: a phase modulator,
having a first phase modulation element configured to generate a first
modulation signal on a first arm in accordance with a first driving
signal, a second phase modulation element for generating a second
modulation signal on a second arm in accordance with a second driving
signal, and a phase shifter configured to provide a phase difference
between the first arm and the second arm, which generates a
phase-modulated signal by combining the first and second modulation
signals; an intensity modulator configured to perform intensity
modulation on the phase-modulated signal output from the phase modulator;
a photodetector configured to generate a power signal representing a
power of an AC component of an optical signal output from the intensity
modulator; first and second amplifiers configured to amplify the first
and second drive signals; a detector configured to perform synchronous
detection on the power-detected signal by using the first low-frequency
signal; a gain adjuster configured to generate a gain adjusting signal
adjusting a level of the first and second drive signals, and a
superimposer configured to superimpose a low-frequency signal on the gain
adjusting signal, wherein the gain adjuster adjusting a gain of at least
one of the first amplifier and the second amplifier based on a result of
the synchronous detection by the detector.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. patent application Ser.
No. 12/640,862, filed Dec. 17, 2009, which is a divisional of U.S. Pat.
No. 7,657,190, issued Feb. 2, 2010, which claims priority to Japanese
Application No. 2006-023111, filed Jan. 31, 2006, the disclosures of all
of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical transmitter apparatus
for generating and transmitting a phase-modulated optical signal, and
particularly relates to an optical transmitter apparatus for generating
and transmitting an optical DQPSK signal.

[0004] 2. Description of the Related Art

[0005] Phase modulation has been in practical use as one of the
technologies for transmitting signals in an optical transmission system.
In the phase modulation, data is transmitted by controlling the phase of
a carrier wave in accordance with the transmission data. For example, in
QPSK (Quadrature Phase Shift Keying) modulation, "θ",
"θ+π/2", "θ+π", and "θ+3π/2" are assigned
respectively to each symbol "00", "10", "11", and "01", consisting of two
bit data. Here, the "θ" is an arbitrary phase. A receiver device
recovers the transmission data by detecting the phase of the received
signal.

[0006] DQPSK (Differential Quadrature Phase Shift Keying) is also known as
a technology, which realizes the QPSK receiver apparatus, with relative
ease. In the DQPSK, the difference between two successive symbols is
assigned with the corresponding phase (0, π/2, π, 3π/2).
Therefore, the receiver device can recover the transmission data by
detecting the phase difference between the two successive symbols.

[0007] FIG. 1 is a diagram describing the configuration of a DQPSK
transmitter apparatus. In this drawing, only necessary configuration is
shown in order to explain the operation principle.

[0008] An optical DQPSK modulator 100 is a Mach-Zehnder modulator, and is
provided with optical continuous wave (CW) generated by an optical source
111. The optical CW is split by an optical splitter, and is guided to a
first arm and a second arm. In the first arm, a phase modulator 101 is
provided, and in the second arm, a phase modulator 102 and a phase
shifter 103 are provided. The phase modulators 101 and 102 are driven by
driving signals data 1 and data 2, respectively. Here, the driving
signals data 1 and data 2 are generated by a data generator unit 112
comprising a DQPSK pre-coder. The phase shifter 103 provides phase
difference π/2 between the first and the second arms. A DQPSK signal
is generated by combining a pair of optical signals output from the phase
modulators 101 and 102.

[0009] FIG. 2 is a diagram explaining the operation of the DQPSK
transmitter apparatus. Here, assume that the phase of the optical
continuous wave is zero.

[0010] The phase of the optical signal output from the phase modulator 101
becomes "0" or "π" in accordance with the driving signal data 1. On
the other hand, the second arm comprises a phase shifter 103 providing
the phase difference of π/2. Therefore, the phase of the optical
signal output from the phase modulator 102 is "π/2" or "3π/2" in
accordance with the driving signal data 2. Then, "π/4", "3π/4",
"5π/4", or "7π/4" is assigned to each symbol with two bit
information (00, 10, 11, 01). The receiver device recovers the
transmission data by detecting the phase difference between successive
two symbols.

[0011] Detailed description of the configuration and the operation of the
optical DQPSK transmitter apparatus is provided in, for example, a Patent
Document 1 (US2004/0081470 or WO2002/051041). In addition, a Patent
Document 2 (US2004/0028418) describes a configuration performing feedback
control of the DC bias of a Mach-Zehnder modulator.

[0012] In order to improve the quality of communications, which employ the
DQPSK (especially, high speed data communication with several ten Gb/s),
the phase of the optical signal output from the optical DQPSK transmitter
apparatus has to be accurately controlled. However, the DQPSK is a
technology under development, and still has a number of problems to be
solved or improved.

[0013] If the amplitude of the driving signals data 1 and data 2 is not
appropriately adjusted, for example, the phase of the DQPSK signal
deviates from a desired phase. In an example shown in FIG. 3, the
amplitude of the driving signal data 1 is smaller than a prescribed
amplitude. In such a case, the intensity of the generated DQPSK signal
also deviates. Note that the amplitude of the driving signal may deviate
due to the aging degradation in the electrical drive system, variations
in characteristics of electronic components or optical components, or
thermal change.

[0014] If the DC bias of the phase modulators 101 and 102 or the amount of
phase shift of the phase shifter 103 is not appropriate, the phase of the
DQPSK signal deviates from the desired phase.

[0015] These problems can occur in the QPSK as well as in the DQPSK.

SUMMARY OF THE INVENTION

[0016] It is an object of the present invention to provide an optical
transmitter apparatus for improving the quality of the communications,
which employ the DQPSK or QPSK.

[0017] The optical transmitter apparatus of the present invention
comprises a phase modulator, having a first phase modulation element for
generating a first modulation signal on a first arm in accordance with a
first driving signal, a second phase modulation element for generating a
second modulation signal on a second arm in accordance with a second
driving signal, and a phase shift element for providing a phase
difference between the first arm and the second arm, which generates a
phase-modulated signal by combining the first and second modulation
signals, an intensity modulator for performing intensity modulation on
the phase-modulated signal output from the phase modulator, a detection
unit for detecting the power of an AC component of an optical signal
output from the intensity modulator, and an adjuster unit for adjusting
an amplitude of at least one of the first driving signal or the second
driving signal so as to minimize the power detected by the detection
unit.

[0018] The phase (and intensity) of the first and second modulation
signals depends on the amplitude of the first and second driving signals
provided to the first and second phase modulation elements. The power of
the AC component of the optical signal output from the intensity
modulator becomes minimum when the amplitude of the first and second
driving signals are optimized. Therefore, a phase-modulated signal with
an appropriate phase corresponding to the first and second driving
signals can be generated by adjusting the amplitude of the first driving
signal and/or the second driving signal so as to minimize the power
value.

[0019] Another aspect of the optical transmitter apparatus of the present
invention comprises a phase modulator, having a first phase modulation
element for generating a first modulation signal on a first arm in
accordance with a first driving signal, a second phase modulation element
for generating a second modulation signal on a second arm in accordance
with a second driving signal, and a phase shift element for providing a
phase difference between the first arm and the second arm, which
generates a phase-modulated signal by combining the first and second
modulation signals, a DC bias generator unit for generating first DC bias
and second DC bias for adjusting an operation point of the first and
second phase modulation elements, an intensity modulator for performing
intensity modulation on the phase-modulated signal output from the phase
modulator, a detection unit for detecting the power of an AC component of
an optical signal output from the intensity modulator, and an adjuster
unit for adjusting an amount of phase shift of the phase shift element as
well as adjusting at least one of the first bias or the second bias based
on the power detected by the detection unit.

[0020] The phase (and intensity) of the first and second modulation
signals depends on the first and second DC bias provided to the first and
second phase modulation elements. The power of the AC component of the
optical signal output from the intensity modulator becomes minimum when
the first bias and the second bias are optimized. Therefore, a
phase-modulated signal with an appropriate phase corresponding to the
first and second driving signals can be generated by adjusting the first
bias and/or the second bias so as to minimize the power value. The
detection unit to detect the power of an AC component of an optical
signal output from the intensity modulator is shared for the adjustment
of the DC bias and the adjustment of the phase shift amount of the phase
shift element. Therefore, the size of the optical transmitter apparatus
can be reduced.

[0021] According to the present invention, it is possible to improve the
quality of communications, which employ the DQPSK or QPSK.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a diagram describing the configuration of a DQPSK
transmitter apparatus;

[0023] FIG. 2 is a diagram explaining the operation of the DQPSK
transmitter apparatus;

[0024] FIG. 3 is a diagram explaining a problem of the conventional DQPSK;

[0025] FIG. 4 is a diagram describing a fundamental configuration of an
optical DQPSK transmitter apparatus relating to the embodiment of the
present invention;

[0026] FIG. 5 is a diagram describing the configuration of an optical
DQPSK transmitter apparatus of the first embodiment;

[0027] FIG. 6A and FIG. 6B show a simulation result on the relation
between the deviation of the amplitude and the power of the monitor
signal;

[0028] FIG. 7 is a diagram explaining the amplitude of the driving signal:

[0029] FIG. 8 is a flowchart showing a process to adjust the amplitude of
the driving signal;

[0030] FIG. 9 is a diagram describing the configuration of the optical
DQPSK transmitter apparatus of the second embodiment;

[0031] FIG. 10A and FIG. 10B show a simulation result on the relation
between the deviation of the DC bias and the power of the monitor signal
in the configuration shown in FIG. 9;

[0032] FIG. 11 is a diagram explaining the DC bias of the phase modulator;

[0033] FIG. 12 is a diagram describing a modification example of the
second embodiment;

[0034] FIG. 13A and FIG. 13B show a simulation result on the relation
between the deviation of the DC bias and the power of the monitor signal
in the configuration shown in FIG. 12;

[0035] FIG. 14 is a diagram describing the configuration of the optical
DQPSK transmitter apparatus of the third embodiment;

[0036] FIG. 15A to FIG. 15C are diagrams explaining the principle of a
method for adjusting the amount of phase shift;

[0037] FIG. 16 is a diagram showing the relation between the f0
component and the phase deviation;

[0038] FIG. 17 is a diagram showing the configuration of the optical DQPSK
transmitter apparatus of the fourth embodiment;

[0039] FIG. 18 is a diagram explaining the synchronous detection by the
synchronous detection unit;

[0040] FIG. 19 is a flowchart showing the operations of the control unit
of the fourth embodiment; and

[0041] FIG. 20 is a diagram describing the configuration of the optical
DQPSK transmitter apparatus of the fifth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] Details of the embodiments of the present invention are set forth
with reference to the drawings. In the following description, a
configuration for generating and transmitting an optical DQPSK signal is
explained; however, the present invention is applicable to an optical
transmitter apparatus for generating and outputting an optical QPSK
signal.

[0043] FIG. 4 is a diagram describing a fundamental configuration of an
optical DQPSK transmitter apparatus relating to the embodiment of the
present invention. Note that the optical DQPSK transmitter apparatus of
the embodiment has a configuration in which RZ intensity modulation is
applied to the generated optical DQPSK signal.

[0044] An optical DQPSK modulator 10 is a Mach-Zehnder modulator, for
example, and an optical continuous wave (CW) generated by an optical
source 111 is provided. The optical CW is split by an optical splitter,
and guided to a pair of arms 11 and 12. Each of the arms 11 and 12 are an
optical waveguide, which propagates an optical signal.

[0045] The arms 11 and 12 comprise phase modulators 13 and 14,
respectively. The phase modulators 13 and 14 are Mach-Zehnder modulators,
for example. Each of the phase modulators 13 and 14 comprises a
modulating electrode. Each modulating electrode is provided with a
corresponding driving signal. The optical path length of the waveguide of
the phase modulators 13 and 14 changes in accordance with the voltage of
the driving signal. Therefore, each of the phase modulators 13 and 14
generates an optical signal with an optical phase corresponding to the
voltage of the provided driving signal. In addition, a phase shifter 15
provides a phase difference π/2 (π/2 +2nπ(n is an integer))
between the arms 11 and 12.

[0046] The phase modulators 13 and 14 are provided with DC bias in order
to adjust their operating points. The DC bias can be provided to the
above modulating electrode or can be provided via a DC bias electrode
comprised separately from the modulating electrode.

[0047] A clock generator unit 112 generates a clock signal. A data
generator unit 113 generates a pair of driving signals, data 1 and data
2, by using the clock signal generated by the clock generator unit 112.
The driving signals data 1 and data 2 are obtained by encoding
transmission data employing a DQPSK pre-coder. The driving signals data 1
and data 2 are provided to the modulating electrodes of the phase
modulators 13 and 14, after being amplified by amplifiers 21 and 22,
respectively. The driving signals data 1 and data 2 are provided to the
modulating electrodes of the phase modulators 13 and 14, respectively, as
differential signals. The amplitude of each differential signal
corresponds to "2Vπ" of the operating characteristics of the phase
modulators 13 and 14. The synchronization between the driving signals
data 1 and data 2 is established by using the clock signal generated by
the clock generator unit 112.

[0048] An RZ intensity modulator 30 performs RZ intensity modulation to
the optical DQPSK signal generated by the optical DQPSK modulator 10. The
RZ intensity modulator 30 is, for example, a Mach-Zehnder modulator
comprising modulating electrodes. The modulating electrodes are provided
with the clock signal generated by the clock generator unit 112 as an
intensity modulating signal. Note that the amplitude of the intensity
modulating signal provided to the RZ intensity modulator 30 corresponds
to "Vπ" of the operating characteristics of the RZ intensity modulator
30.

[0049] A photodetector 16 converts the optical DQPSK signal generated by
the optical DQPSK modulator 10 into an electrical signal. A photodetector
31 converts the optical RZ-DQPSK signal generated by the RZ intensity
modulator 30 into an electrical signal. In this embodiment, each of the
photodetectors 16 and 31 is a photodiode, for example. The signal
obtained by the photodetectors 16 and 31 is used as a feedback signal for
controlling the operation of the optical DQPSK transmitter apparatus.

[0050] In the optical DQPSK transmitter apparatus with the above
configuration, the operation of the optical DQPSK modulator 10 is the
same as explained with reference to FIG. 2. In other words, the phase
(π/4, 3π/4, 5π/4, or 7π/4) of the optical DQPSK signal
generated by the optical DQPSK modulator 10 is determined in accordance
with a pair of the driving signals data 1 and data 2 (00, 10, 11, or 01)
corresponding to each symbol. The speed of the data transmitted by the
optical DQPSK signal is not limited in particular; however, it is several
ten Gb/s.

[0051] The RZ intensity modulator 30 generates the optical RZ-DQPSK signal
by RZ intensity modulation on the optical DQPSK signal. The optical
RZ-DQPSK signal is obtained by reducing the optical power of the DQPSK
signal in a period equivalent to a symbol rate of the transmission
signal. In other words, the optical power of the RZ-DQPSK signal
fluctuates at a frequency, which is the same as the symbol rate of the
transmission signal. It should be noted that the phase of a signal
carrier wave does not change in association with the RZ modulation. In
other words, the phase of each symbol of the RZ-DQPSK signal is "π/4",
"3π/4", "5π/4", or "7π/4".

First Embodiment

[0052] FIG. 5 is a diagram describing the configuration of an optical
DQPSK transmitter apparatus of the first embodiment. The optical DQPSK
transmitter apparatus of the first embodiment provides a function for
adjusting the amplitude of a driving signal based on the power of the
optical RZ-DQPSK signal generated by the RZ intensity modulator 30. In
FIG. 5, descriptions of the clock generator unit 112 and the data
generator unit 113 are omitted.

[0053] The photodetector 31 converts the optical RZ-DQPSK signal output
from the RZ intensity modulator 30 into an electrical signal (hereinafter
referred to as a "monitor signal"). A capacitor C removes a DC component
from the monitor signal obtained by the photodetector 31. By so doing, an
AC component of the optical RZ-DQPSK signal is extracted. An amplifier 41
amplifies the monitor signal from which the DC component is removed. A
power detector unit 42 detects power of the monitor signal amplified by
the amplifier 41. The power of the monitor signal is detected by
averaging (i.e. integration) the voltage of the monitor signal.

[0054] As described above, the monitor unit, comprising the photodetector
31, the capacitor C, the amplifier 41, and the power detector unit 42,
detects the power of the monitor signal corresponding to the optical
RZ-DQPSK signal. In this embodiment, the bandwidth of the monitor unit
(the upper limit operating speed) is not limited in particular; however
it can be several ten MHz to several GHz, for example.

[0055] A minimum power detector unit 43 and a gain adjuster unit 44 adjust
the gains of the amplifiers 21 and 22 so that the power of the monitor
signal detected by the power detector unit 42 is to be minimum. The
minimum power detector unit 43 and the gain adjuster unit 44 can be
implemented by a CPU executing a prepared program, for example.

[0056] FIG. 6A and FIG. 6B show a simulation result on the relation
between the deviation of the amplitude of the driving signal and the
power of the monitor signal. Here, FIG. 6A shows the relation between the
deviation of the amplitude of the driving signal data 1 provided to the
phase modulator 13 of the arm 11 (I arm) and the power of the monitor
signal under an assumption that the amplitude of the driving signal data
2 provided to the phase modulator 14 of the arm 12 (Q arm) is
appropriately adjusted. Similarly, FIG. 6B shows the relation between the
deviation of the amplitude of the driving signal data 2 provided to the
phase modulator 14 of the Q arm and the power of the monitor signal under
an assumption that the amplitude of the driving signal data 1 provided to
the phase modulator 13 of the I arm is appropriately adjusted.

[0057] The appropriate amplitude of the driving signals data 1 and data 2
is, basically, equivalent to 2Vπ of the operation characteristics of
the phase modulators 13 and 14, as shown in FIG. 7. When each of the
amplitude of the driving signals data 1 and data 2 is optimized, the
phase of the optical DQPSK signal is accurately assigned, and
communication quality is improved. Therefore, in this simulation, the
calculation is performed on the basis of the amplitude of the driving
signals data 1 and data 2 being 2Vπ. Note that if the amplitude of the
driving signals deviates from 2Vπ, the optical intensity of the
corresponding phase modulator is changed. For that reason, when the
amplitude of the driving signal deviates from an optimal value (or when
the amplitude of a pair of driving signals becomes imbalanced), the phase
(and intensity) of the optical DQPSK signal is in an inappropriate state,
causing the deterioration of the communication quality, as explained with
reference to FIG. 3.

[0058] As shown in FIG. 6A and FIG. 6B, the characteristic of the I arm
and the Q arm are mostly the same. The above simulation result indicates
that in either the I arm and the Q arm, the power of the monitor signal
is minimized when the amplitude of the driving signals data 1 and data 2
are appropriately adjusted. Therefore, by adjusting the amplitude of the
driving signals data 1 and data 2 so that the power of the monitor signal
is minimized, the communication quality can be improved.

[0059] It should be noted that, as shown in FIG. 6A and FIG. 6B, when the
bandwidth of the monitor unit (the photodetector 31, the capacitor C, the
amplifier 41, and the power detector 42) is low, the power of the monitor
signal detected by the power detector 42 becomes small. However, even if
the bandwidth of the monitor unit is lowered, the amount of change in the
power of the monitor signal with respect to the deviation of the
amplitude of the driving signals is approximately the same. For example,
in either case of the bandwidth of the monitor unit being 1 GHz or being
100 MHz, if the amplitude of the driving signal deviates by
0.1×Vπ, the power of the monitor signal changes by approximately
12 dB. Thus, even if the bandwidth of the monitor unit is lowered, it is
possible to optimize the amplitude of the driving signals data 1 and data
2 based on the power of the monitor signal.

[0060] The reason why the power of the monitor signal is changed in
accordance with the amplitude of the driving signal is considered to be
caused by the change in the distance between the origin and the signal
point of the optical DQPSK signal on the phase plane and change in
optical power of the optical DQPSK signal, as explained with reference to
FIG. 3.

[0061] The above simulation is performed under an assumption that the
amount of phase shift of the phase shifter 15 is accurately adjusted to
π/2. If the amount of phase shift of the phase shifter 15 deviates
from π/2 in the simulation, the amount of change in the power of the
monitor signal with respect to the deviation of the amplitude becomes
small; however, the similar characteristics can be obtained. In other
words, even when the amount of phase shift of the phase shifter 15 is not
accurately adjusted to π/2, the amplitude of the driving signal can be
optimized by adjusting the gains of the amplifiers 21 and 22 so as to
minimize the power of the monitor signal. Therefore, it is possible to
optimize the amplitude of the driving signal before adjusting the amount
of phase shift of the phase shifter 15, and it is also possible to
accurately adjust the amount of phase shift of the phase shifter 15.

[0062] FIG. 8 is a flowchart showing a process to adjust the amplitude of
the driving signal. The process is executed for each of the I arm and the
Q arm.

[0063] In steps S1 through S6, the power value of the monitor signal is
obtained while changing the gain of the amplifier 21 (or 22).
Specifically, the power value A obtained with the current gain, the power
value B obtained with the gain smaller than the current gain by a
prescribed value, and the power value C obtained with the gain larger
than the current gain by a prescribed value are obtained and stored.

[0064] In steps S7 through S10, the power values A, B, and C are compared
with one another. When "A<B" and "A<C" (step S7: Yes), it is
determined that the power of the monitor signal obtained with the current
gain is the minimum value, and the process is terminated. When "A>B"
and "A<C" (step S8: Yes), the gain of the amplifier 21 is reduced by a
prescribed value in step S11, and the process returns to step S1. When
"A<B" and "A>C" (step S9: Yes), the gain of the amplifier 21 is
increased by a prescribed value in step S12, and the process returns to
step S1. In the case of the "A>B" and "A>C", it is considered as an
error state, however the gain of the amplifier 21 is reduced by a
prescribed value in step S13, and the process returns to step S1.

[0065] The above process is executed alternately in the I arm and the Q
arm and is repeated, for example. By so doing, the amplitude of the
driving signal is optimized in both of the I arm and the Q arm.

[0066] The above process should be executed in both of the I arm and the Q
arm preferably; however, execution of the above process in either one of
the I arm or the Q arm can still improve the communication quality.

Second Embodiment

[0067] FIG. 9 is a diagram describing the configuration of the optical
DQPSK transmitter apparatus of the second embodiment. The optical DQPSK
transmitter apparatus of the second embodiment provides a function for
adjusting the DC bias of each of the phase modulators 13 and 14 in the
optical DQPSK modulator 10, based on the power of the optical RZ-DQPSK
signal generated by the RZ intensity modulator 30. Note that the
descriptions of the clock generator unit 112 and the data generator unit
113 are omitted.

[0068] In FIG. 9, the monitor unit (i.e. the photodetector 31, the
capacitor C, the amplifier 41 and the power detector 42) is basically the
same as that of the first embodiment. In other words, the monitor unit
detects the power of the optical RZ-DQPSK signal and generates a monitor
signal.

[0069] A minimum power detector unit 51 and a bias adjuster unit 52 adjust
the DC bias for adjusting the operation point of the phase modulators 13
and 14 so that the power of the monitor signal detected by the power
detector unit 42 becomes minimum. Here, the minimum power detector unit
51 and the bias adjuster unit 52 are realized by the CPU executing a
prepared program, for example. A DC bias generator unit 53 generates the
DC bias in accordance with the instruction from the bias adjuster unit 52
and provides the DC bias to the phase modulators 13 and 14.

[0070] FIG. 10A and FIG. 10B show a simulation result on the relation
between the deviation of the DC bias and the power of the monitor signal.
FIG. 10A shows the relation between the deviation of the DC bias provided
to the phase modulator 13 of the arm 11 (I arm) and the power of the
monitor signal under an assumption that the DC bias provided to the phase
modulator 14 of the arm 12 (Q arm) is appropriately adjusted. Similarly,
FIG. 10B shows the relation between the deviation of the DC bias provided
to the phase modulator 14 of the Q arm and the power of the monitor
signal under an assumption that the DC bias provided to the phase
modulator 13 of the I arm is appropriately adjusted.

[0071] A proper value of the DC bias is as shown in FIG. 11, and it is a
value at which a state that the driving signal dithers about the point
where the optical output intensity of the phase modulator is local
minimum is obtained. The simulation shown in FIG. 10A and FIG. 10B
calculates the case when the DC bias of the phase modulator deviates from
the proper value. When the DC bias deviates from the proper value, the
intensity of the output optical signal from corresponding phase modulator
is changed. Therefore, when the DC bias of each phase modulator deviates
from the proper value (or the DC bias of a pair of the phase modulators
becomes imbalanced), the phase (and intensity) of the optical DQPSK
signal is in an inappropriate state, causing the deterioration of the
communication quality. Note that the operation point of the phase
modulator may fluctuate due to the DC drift or thermal change etc.

[0072] As shown in FIG. 10A and FIG. 10B, the characteristics of the I arm
and the Q arm are mostly the same. The above simulation result indicates
that in either the I arm and the Q arm, the power of the monitor signal
is minimized when the DC bias of the phase modulators 13 and 14 are
appropriately adjusted. Therefore, by adjusting the DC bias so that the
power of the monitor signal is minimized, the communication quality can
be improved. Here, the procedure of the process minimizing the power of
the monitor signal is basically based on the flowchart of FIG. 8.

[0073] It should be noted that in the example shown in FIG. 9, the monitor
signal is generated by using the optical RZ-DQPSK signal output from the
RZ intensity modulator 30. The monitor signal, also, may be generated by
using the optical DQPSK signal output from the optical DQPSK modulator
10. In such a case, the monitor signal obtained from the photodetector 16
is transmitted to the power detector 42 via the capacitor C and the
amplifier 41. However, as shown in FIG. 10A and FIG. 10B, the change in
the power of the monitor signal obtained by using the optical DQPSK
signal is smaller than the change in the power of the monitor signal
obtained by using the optical RZ-DQPSK signal. Therefore, in terms of the
DC bias adjustment, the configuration in which the monitor signal is
generated by using the optical RZ-DQPSK signal has advantage over the
configuration using the optical DQPSK signal. Additionally, in a case
that the optical DQPSK modulator 10 and the RZ intensity modulator 30 are
formed on one substrate, the configuration in which the optical output of
the RZ intensity modulator 30 is monitored has advantage for
miniaturization.

[0074] FIG. 12 is a diagram describing a modification example of the
second embodiment. The optical DQPSK transmitter apparatus shown in FIG.
9 comprises the capacitor C for removing DC component of the monitor
signal obtained by the photodetector 31, and controls the DC bias of the
phase modulators 13 and 14 by using the AC component of the optical
RZ-DQPSK signal. On the other hand, the optical DQPSK transmitter shown
in FIG. 12 does not comprise the capacitor C for removing the DC
component of the monitor signal. In other words, the monitor signal
containing the DC component is transmitted to the power detector unit 42.

[0075] A maximum power detector unit 54 and a bias adjuster unit 55 adjust
the DC bias for adjusting the operation point of the phase modulators 13
and 14 so that the power of the monitor signal detected by the power
detector unit 42 becomes maximum. The DC bias generator unit 53 generates
DC bias in accordance with the instruction from the bias adjuster unit 55
and provides the DC bias to the phase modulators 13 and 14.

[0076] FIG. 13A and FIG. 13B show a simulation result on the relation
between the deviation of the DC bias and the power of the monitor signal.
Note that FIG. 13A and FIG. 13B shows the simulation result of the I arm
and the Q arm, respectively, under the same conditions as the FIG. 10A
and FIG. 10B.

[0077] The above simulation result shows that, in either of the I arm and
the Q arm, the power of the monitor signal becomes maximum when the DC
bias of the phase modulators 13 and 14 is appropriately adjusted.
Therefore, in such a case, by adjusting the DC bias so as to maximize the
power of the monitor signal, the communication quality can be improved.
In the configuration shown in FIG. 12 (the configuration in which the DC
component of the monitor signal is not removed), there is no large
difference between the case of generating the monitor signal from the
optical DQPSK signal and the case of generating the monitor signal from
the optical RZ-DQPSK signal, as shown in FIG. 13A and FIG. 13B.

Third Embodiment

[0078] FIG. 14 is a diagram describing the configuration of the optical
DQPSK transmitter apparatus of the third embodiment. The optical DQPSK
transmitter apparatus of the third embodiment comprises a function for
adjusting the amount of phase shift of the phase shifter 15 of the
optical DQPSK modulator 10 in addition to the DC bias control function of
the second embodiment.

[0079] A control unit 60 provides the DC bias control function and the
phase shift amount adjusting function based on the power of the monitor
signal obtained by the power detector unit 42. Here, the control unit 60
is realized by a CPU, for example, executing a prepared program. A gain
adjuster unit 61 is the same as the minimum power detector unit 51 and
the bias adjuster unit 52 shown in FIG. 9.

[0080] The amount of phase shift of the phase shifter 15 has to be
accurately adjusted at π/2, as stated above. The amount of phase shift
of the phase shifter 15, particularly in this embodiment, is adjusted by
the DC bias applied to the phase shifter 15.

[0081] A DC bias generator unit 62 generates a DC bias signal for
adjusting the amount of phase shift of the phase shifter 15 in accordance
with the instruction from a bias adjuster unit 66. A pilot signal
generator unit 63 generates a pilot signal. The frequency of the pilot
signal (hereinafter referred to as f0) is adequately lower than the bit
rate of the transmission data, and, for example, it is from a several kHz
to a several MHz. The generated pilot signal is superimposed on the DC
bias signal and provided to the phase shifter 15. By so doing, the amount
of phase shift of the phase shifter 15 changes at the frequency f0. As a
result, the optical DQPSK signal, the optical RZ-DQPSK signal, and the
monitor signal contain the f0 component and its harmonic component.

[0082] A band pass filter 64 extracts the f0 component. In other words,
the f0 component is extracted from the power signal representing the
power of the monitor signal. A synchronous detection unit 65 performs
synchronous detection on the output signal of the band pass filter 64
using the pilot signal generated by the pilot signal generator unit 63. A
bias adjuster unit 66 determines the DC bias for adjusting the amount of
phase shift to be generated by the bias generator unit 61 in accordance
with the result of the synchronous detection.

[0083] A method for adjusting the amount of phase shift of the phase
shifter 15 is set forth with reference to FIG. 15A through FIG. 15C. The
power detected by the power detector unit 42, as shown in FIG. 15A,
depends on the amount of phase shift of the phase shifter 15, and the
power is minimum when the amount of phase shift is "π/2".

[0084] When the pilot signal is provided to the phase shifter 15, the
amount of phase shift of the phase shifter 15 changes (or oscillates) by
the frequency f0. At that time, if the amount of phase shift is "π/2"
or close to "π/2", the amount of phase shift, when the pilot signal is
provided, dithers about the point where the power detected by the power
detector unit 42 is minimum. For example, during one cycle period of the
pilot signal (time t1-t5), the power detected by the power detector unit
42 changes as the following: Time t1, approximately +30 dB (relative
value); time t2, zero (relative value); time t3, approximately +30 dB
(relative value); time t4, zero (relative value); and time t5,
approximately +30 dB (relative value). In other words, the power detected
by the power detector unit 42 oscillates at the frequency 2f0,as shown in
FIG. 15B. That is, the output signal of the power detector unit 42
contains the 2f0component.

[0085] Meanwhile, if the amount of phase shift deviates from the "π/2",
the amount of phase shift, when the pilot signal is provided, dithers in
a region away from the point where the power detected by the power
detector unit 42 is minimum. For example, during the time t1-t5, the
power detected by the power detector unit 42 changes as the following:
time t1, approximately +36 dB (relative value); time t2, approximately
+39 dB (relative value); time t3, approximately +42 dB (relative value);
time t4, approximately +39 dB (relative value); and time t5,
approximately +36 dB (relative value). In this case, the output signal of
the power detector unit 42 contains the f0 component as shown in FIG.
15C; however, the 2f0 component is not contained.

[0086] As explained above, when the amount of phase shift deviates from
"π/2", the f0 component is detected from the output signal of the
power detector unit 42; however, the 2f0 component is not detected. When
the amount of phase shift is close to "π/2", the 2f0 component is
detected from the output signal of the power detector unit 42, while the
f0 component is reduced. Therefore, the amount of phase shift can be
converged at "π/2" by the feedback control on the phase shifter 15 so
that the f0 component detected by the synchronous detection unit 65
becomes minimum.

[0087] In addition, the phase of the f0 component signal output from the
power detector unit 42 when the amount of the phase shift deviates in a
positive side is inverted from the phase when the amount of the phase
shift deviates in a negative side. Therefore, the bias adjuster unit 66
can determine whether the amount of the phase shift by the phase shifter
15 is to be increased or to be decreased (i.e. whether the phase is to be
rotated in a positive direction or in a negative direction) based on the
phase of the f0 component signal detected by the synchronous detection
unit 65. The output signal of the synchronous detection unit 65 is shown
in FIG. 16. Note that the method for adjusting the amount of phase shift
of the phase shifter by using the pilot signal are described, for
example, in U.S. patent application Ser. No. 11/298,892.

[0088] As explained above, the optical DQPSK transmitter apparatus of the
third embodiment provides a DC bias control function and a phase shift
amount adjusting function. The monitor unit for generating the monitor
signal (i.e. the photodetector 31, the capacitor C, the amplifier 41 and
the power detector unit 42) can be shared for realizing both of the two
functions. Consequently, increase in the circuit size can be suppressed.
Note that the DC bias control process and the phase shift amount
adjusting process may be executed simultaneously in parallel or may be
executed separately.

Fourth Embodiment

[0089] FIG. 17 is a diagram showing the configuration of the optical DQPSK
transmitter apparatus of the fourth embodiment. The optical DQPSK
transmitter apparatus of the fourth embodiment comprises a function for
adjusting the amplitude of the driving signal by employing the pilot
signal.

[0090] A control unit 70 adjusts the gains of the amplifiers 21 and 22
based on the monitor signal power value obtained by the power detector
unit 42 in order to optimize the amplitude of the driving signal data 1
and data 2. Here, the control unit 70 can be realized by a CPU executing
a prepared program, for example.

[0091] A pilot signal generator unit 71 generates the pilot signal for
adjusting the gains of the amplifiers 21 and 22. The frequency of the
pilot signal (hereinafter referred to as f1) is substantially low,
compared with the bit rate of the transmission data, and it is several
kHz to several MHz, for example. The generated pilot signal is
superimposed on each of a pair of gain control signals for controlling
the gains of the amplifiers 21 and 22, and is provided to the amplifiers
21 and 22. By so doing, the gain of the amplifiers 21 and 22 changes by
the frequency f1, and the amplitude of the driving signal data 1 and data
2 changes by the frequency f1. As a result, the optical DQPSK signal, the
optical RZ-DQPSK signal, and the monitor signal contain the f1 component
and its harmonic component.

[0092] A synchronous detection unit 72 performs synchronous detection on
the power signal representing the power of the monitor signal obtained by
the power detector unit 42 by using the pilot signal (or a
frequency-doubled signal, the frequency of which is increased twice as
high as that of the pilot signal) generated by the pilot signal generator
unit 71. The gain adjuster unit 73 generates a gain adjusting signal for
adjusting the gains of the amplifiers 21 and 22 in accordance with the
result of the synchronous detection. A bandpass filter 74 for extracting
the f1 component (or 2f1 component) from the power signal representing
the power of the monitor signal may be provided in the previous stage of
the synchronous detection unit 72.

[0093] FIG. 18 is a diagram explaining the synchronous detection by the
synchronous detection unit 72. The output of the synchronous detection
unit 72 is zero (or approximately zero) when the amplitude of the driving
signals data 1 and data 2 is optimized. Thus, the amplitude of the
driving signal is optimized by adjusting the gains of the amplifier 21
and 22 so that the output of the synchronous detection unit 72 converges
at zero. The polarity (positive or negative) of the output of the
synchronous detection unit 72 changes depending on whether the amplitude
of the driving signal is larger or smaller than the optimal value.
Therefore, whether the gains of the amplifiers 21 and 22 should be
increased or should be reduced can be recognized by monitoring the
polarity of the output of the synchronous detection unit 72.

[0094] FIG. 19 is a flowchart showing the operations of the control unit
70. In step S21, the synchronous detection signal output from the
synchronous detection unit 72 is obtained. In step S22, whether or not
the synchronous detection signal is zero (or approximately zero) is
checked. Here, the "approximately zero" indicates that the signal is
smaller than the expected noise etc. If the synchronous detection signal
is zero (or approximately zero), it is determined that the amplitude of
the driving signal is adjusted to the optimal value, and the process is
terminated.

[0095] In steps S23 and S24, the polarity of the synchronous detection
signal is checked. When the polarity of the synchronous detection signal
is positive, it is determined that the amplitude of the driving signal is
too large, and an instruction to reduce the gain of the amplifier 21 (or
22) by a prescribed amount in step S25. On the other hand, when the
polarity of the synchronous detection signal is negative, it is
determined that the amplitude of the driving signal is too small, and the
instruction to increase the gain of the amplifier 21 (or 22) by a
prescribed amount in step S26. The processes in steps S23 to S26 are
repeated until the synchronous detection signal converges at zero (or
approximately zero).

[0096] The processes of the flowchart are performed at initialization of
the optical DQPSK transmitter apparatus, and the processes may be
additionally performed during the optical DQPSK transmitter apparatus is
in operation, if needed.

Fifth Embodiment

[0097] FIG. 20 is a diagram describing the configuration of the optical
DQPSK transmitter apparatus of the fifth embodiment. The optical DQPSK
transmitter apparatus of the fifth embodiment comprises a function for
adjusting the amount of phase shift of the phase shifter 15 in addition
to the amplitude adjusting function of the forth embodiment.

[0098] A control unit 80 comprises the DC bias generator unit 62, the
pilot signal generator unit 63, the synchronous detection unit 65, and
the bias adjuster unit 66, which are explained with reference to FIG. 14,
further comprises the pilot signal generator unit 71, the synchronous
detection unit 72, and the gain adjuster unit 73, which are explained
with reference to FIG. 18, and provides the amplitude adjusting function
and the phase shift amount adjusting function. Note that the explanation
of the operations of the control unit 80 is omitted in this section since
the same operations are explained in the third and the fourth
embodiments.

[0099] It is preferable that the frequency of the pilot signal for
adjusting the amplitude generated by the pilot signal generator unit 71
and that of the pilot signal for adjusting the amount of phase shift
generated by the pilot signal generator unit 63 differ from each other.
Particularly, in the case that the amplitude adjusting process and the
phase shift amount adjusting process is performed in parallel, those
frequencies have to be different from each other.

Variation

[0100] The adjusting processes of the first through the fifth embodiments
are performed at initialization of the optical DQPSK transmitter
apparatus, and the processes may be additionally performed during the
optical DQPSK transmitter apparatus is in operation, if needed. When
performing the adjusting processes during the optical DQPSK transmitter
apparatus is in operation, the processes may be periodically performed,
or may be continually performed.

[0101] The first, the forth and the fifth embodiments describes the
configurations for optimizing the amplitude of the driving signal by
adjusting the gain of the amplifier for amplifying the driving signal;
however, the present invention is not limited to such configurations. In
other words, it is possible that a variable attenuator for adjusting the
amplitude of the driving signal is provided instead of the amplifiers 21
and 22 and the amount of attenuation of the variable attenuator is
adjusted by according to the monitor signal.

[0102] In the above embodiments, the DQPSK modulation is explained in
main; however, the control of the present invention is applicable to the
QPSK modulation. The present invention is also applicable to the 2nPSK
(n≧3) or QAM. However, in applying the present invention to these
modulations, multiple-value data should be used as a data signal input to
a data modulator unit, for example.

[0103] When an LN (LiNbO3) modulator is employed as a modulator, it is
preferable to use a polarization maintaining optical fiber for the
connection between the optical source and the optical DQPSK modulator,
and between the optical DQPSK modulator and the RZ intensity modulator,
since the LN modulator has a polarization dependency.